Brexit and import duty

I can't seem to find any information about whether my pyra purchase will be subject to import duty after March 29th. I understand that this is because no-one has a clue what will happen due to political ineptitude but if I could avoid paying more then I will.

Would it be possible to pay the outstanding amount now and get a receipt which proves the goods were bought before March 29th? This should be enough to argue that the purchase was made while we (Brits) were still part of the EU and that the pyra would then be exempt from import duty.

As far as I know, it's the duty of someone sending a package to put the value of the goods inside regardless of when they were actually paid for.

Assuming it'll be similar to how buying anything from the US is now, it cost me about 40 pounds when I received someone prepaid via kickstarter that got picked up by customs. That was a much cheaper device, but about 10 quid of that came from the VAT that needed to be added to the order. IIRC the post office charged me about 30 quid for dealing with the package. If the Pyra comes via a courier like my Pandora did (it was DPD as I recall) then it might cost more or less, but I wouldn't know about that.

I guess we need to look up what WTO rules charge for electronic goods. My package from america was electronic components, but none of that charge as I can recall was classified as a customs charge, I only paid VAT and a post office charge.

I don't think, customs care so much for when you gave your money away as for when you import something and what that thing is worth. For the latter they go by what you have paid, of course.
With German customs you have a threshold of 150€ for import fees (aside from import tax, which hase a lower threshold). So if your import cost including the cost to get it here pass that threshold, import fees are due.
And they go so far - in case you paid with another currency than EUR - to go by the exchange rate at the time they check, not the time you did pay. Which I find very annoying. How should I know, what the rate will be, when my shipment reaches customs.